The present invention pertains to electrochemical cells and the collection of electrical current produced by electrodes. In particular the invention is an electrochemical cell and current collector providing an improved electrical connection between electrodes and cell terminals.
In electrochemical cells it is necessary to collect the electrical current produced in the electrodes and deliver it to the cell terminals where it can be directed to external use. Cells include circuitry between the electrodes and the terminals for this purpose. Particularly in cells having a high discharge rate, the effectiveness of this circuitry is important to performance. A critical element of this circuitry is a current collector that collects the current produced over the entire area of the electrode. The current collector must then deliver the current to other circuit elements or a cell terminal. The effectiveness of the current collector is exhibited by low internal resistance. Where current collection is not effectively performed, the cell will generally exhibit higher resistance to current passage, reduced discharge rates and reduced output voltage. Higher operating temperatures are also a possible consequence.
Many electrochemical cells, particularly secondary cells such as nickel-cadmium and nickel-metal hydride cells, employ discrete electrodes that are each formed of a thin conductive substrate on which an electrochemically active material is deposited. Because the effective volume of the active material is dispersed over the relatively large area of the substrate, collection of generated current from the entire substrate is problematic. This problem may be increased by the manner in which the electrodes are shaped or stacked to fit within the cell container. In what is known as a "jelly-roll" configuration, positive and negative electrodes are wound in a spiral fashion for insertion into a round container. One form of a current collector used on electrodes in jelly-roll assemblies, as well as other cells, is a small collector tab welded or otherwise attached to the edge of the electrode substrate. These tabs are small with respect to the electrode substrate and contact the electrode at a single discrete location. Consequently, the current path for much of the electrode to this collector is relatively long resulting in high resistance. A conductive strap is typically used to connect the tab to the cell terminal. Fabrication problems also exist with these edge-welded tabs; welding of the tabs is burdensome for high volume production. As well, post-assembly welding of the straps to the cell container or terminal is also difficult.
These problems are somewhat addressed for jelly-roll cell assemblies by using collectors which attempt a pressure contact on the entire coiled edge of the electrodes at the end of the jelly-roll. By winding the electrodes with one electrode axially offset from the other, each end of the jelly-roll assembly presents a single spiral electrode edge. One spiral edge (generally the negative electrode) is located adjacent the bottom of the cell container and the other (generally the positive) adjacent the top. A current collector is then placed between the end of the container and the respective electrode spiral edge. The current collector is configured to contact the spiral electrode edge at a multitude of points increasing the effective current path area. A version of this current collector is presented in U.S. Pat. No. 4,529,675 to Sugalski. The Sugalski collector introduces resilient deflected tabs biased to maintain contact with the electrode edge. However, contact with the electrode is limited in this construction to the edges of the tabs. In addition, because the tabs' edges present a rigid edge and are preset in shape and orientation they are unable to accommodate, or adjust to, possible variations in offset or shape of the spiraled edge. Although the tabs are resilient, their contacting edges are rigid with respect to the electrode edge and therefore cannot adjust to variations in electrode height. The inability to adjust to variations in shape is shared by all current collectors which are locally rigid.
In delivering the collected current to the cell container or terminal, current collectors such as that presented in the Sugalski patent also suffer from their inherent rigidity. Collectors that incorporate an effectively rigid body, such as the base portion of Sugalski's collector, generally do not make effective pressure contact against flat surfaces such as the bottom of a cell container. Because of the rigidity of the structures, contact at a small number of points usually results. To prevent high resistance to current flow a welded connection to the container is often necessary.
An additional disadvantage of the Sugalski device and similar devices is the relatively large volume of the cell container required for their use. Commercial cell dimensions are typically fixed by industry standard configurations. For a particular configuration, maximizing capacity requires minimizing the space or volume taken up by non-active elements such as current collectors. To minimize non-active volume, an optimum current collector will have as short or low a profile as possible. What is needed is an improved current collector that enables a low resistance connection to electrodes by establishing contact on a maximum area of the electrode. The same electrode should provide a low resistance connection to the cell container, be easy to assemble, and have a low profile.